The present invention relates generally to rotary machines, and, more particularly, to methods and apparatus to facilitate sealing between rotary and stationary components within the rotary machine.
Steam and gas turbines are used, among other purposes, to generate power for electric generators. A steam turbine has a steam path which typically includes, in serial-flow relationship, a steam inlet, a turbine, and a steam outlet. A gas turbine has a gas path which typically includes, in serial-flow relationship, an air intake (or inlet), a compressor, a combustor, a turbine, and a gas outlet (or exhaust nozzle). Compressor and turbine sections include at least one row of circumferentially spaced rotating blades or buckets.
Turbine efficiency depends at least in part on a radial clearance or gap between tips of the rotating buckets and the surrounding casing, and maintaining a clearance between the rotor and the diaphragm packings. If either clearance is too large, steam or gas flow may leak through the clearance gaps, thus decreasing the turbine's efficiency. Alternatively, if either clearance is too small, the rotor bucket tips may undesirably contact the surrounding casing during certain turbine operating conditions. Gas or steam leakage, either out of the flow path or into the flow path, from an area of higher pressure to an area of lower pressure, is generally undesirable.
To facilitate minimizing flow-path leakage, at least some known turbines use a plurality of labyrinth seals. Known labyrinth seals include longitudinally spaced-apart rows of labyrinth seal teeth to facilitate sealing against high-pressure differentials that may be present in a turbine. However, anticipated relative axial movements between rotary and stationary components may exclude the use of labyrinth seals when covers are peened across tenons extending from the bucket tips. In addition, centrifugal stresses induced in the covers and tenons may limit the size and amount of sealable contact to a single seal member positioned along the exit side of the blade. However, such seal configurations may have a relatively high flow coefficient in comparison to other labyrinth seal arrangements. Furthermore, single seal members may also be more sensitive to relative axial movements between rotary and stationary components. Alternatives to single exit side seals are tip brush seals and integral cover buckets, however, both of these alternatives are generally more costly and may be far less reliable than the single seal member arrangement.